Undine, also sometimes rendered as Ondine, is the Water Spirit and one of the Four Great Spirits in the world of Black Clover. She takes the form of a beautiful woman whose long hair and gown are made entirely of water, with fish-like fins in place of her ears, blue eyes, and pale skin that appears almost blue. Her body is essentially composed of water, reflecting her identity as a pure elemental being tied to that element.

In terms of background, Undine is one of the four great elemental spirits that govern the natural elements. She has a long tradition of serving the rulers of the Heart Kingdom, having formed a contract with the first queen of the newly established kingdom and acting as its guardian ever since. Across generations, she has bonded with the princesses and queens of the Heart Kingdom, and her current contract holder is Loropechika, the Queen of the Heart Kingdom.

Her personality can be misleading at first. She often comes across as smug, condescending, cool, and even provocative, especially toward strangers or those she considers beneath her. Beneath that sharp exterior, however, she is genuinely warm and caring. In reality, she is motherly and doting, particularly when it comes to her contractor. Because she sees Loropechika as beautiful and overflowing with talent, she constantly tries to look after her, protect her, and support her in every way. Her temperament is noted to be quite similar to that of her fellow spirit Sylph, the Wind Spirit.

Undine's central motivation is the protection of the Heart Kingdom and the wellbeing of Loropechika. When the queen was afflicted with a powerful curse by a devil, Undine devoted herself to finding a way to break it. This goal drives her to aid in the effort to defeat the devil responsible, making her a determined and loyal force in the fight against the demonic threats that endanger the kingdom.

Within the story, Undine acts as both a guardian and a test of strength for visiting allies. When members of the Clover Kingdom's Magic Knights come to the Heart Kingdom for training, she participates in trials designed to measure their abilities. In one such instance, she creates multiple armored water knights to attack Asta and, when he cuts through them, responds by trapping him in a whirlpool and attempting to drown him by restraining his wrists to the floor. Beyond these tests, she fights alongside the queen and the allied mages in the battles against the Spade Kingdom and its devil forces, lending her immense water power to their defense.

Her key relationships center on Loropechika, whom she treasures and treats with deep affection and care. She also interacts with the mages who come to the Heart Kingdom as guests, including water magic users from the Clover Kingdom, and shares a notable kinship with the other great spirits such as Sylph. Over the course of these events, she grows more trusting of the outsiders who prove their worth, working with them to safeguard the queen and break her curse.

Undine's notable abilities come from her Water Magic, which allows her to generate and manipulate water in a wide variety of forms. She can shape water into offensive constructs such as armored knights, create copies of herself, summon powerful whirlpools to trap and crush enemies, cast protective barriers, and restrain opponents with water-based bindings. As one of the Four Great Spirits, her magical power is vast, making her one of the most formidable elemental beings in the series and a crucial asset in the conflicts surrounding the Heart Kingdom.